In applications with stringent space and thermal constraints, it is highly desirable to provide high power density motor-generators that are compact and capable of operating with high temperature ranges. This requires that space utilization must be maximized in order for the motor-generator to provide maximum possible torque and power outputs under all application needs as well as high ambient temperatures. Belt-driven air-cooled motor-generators today are generally equipped with axial flow fans to provide force air-cooling so that these electric machines can operate at high power and high ambient temperatures. The axial flow fans are typically disposed axially and mounted on one end of the rotor shaft outside of the housing enclosures. The arrangement of an axial flow fan on the shaft of a motor-generator allows air at lower ambient temperatures to be pulled axially from one end of the housing enclosure through the entry vents so that it flows over the motor-generator stator stack and windings and exhausts through the exit vents on the other side of the housing enclosure.
While having an axial flow fan mounted on one end of the rotor shaft to provide airflow and cooling in order to enable an electric motor-generator to be operated at high power levels and high ambient temperatures, it is however using valuable space that can be effectively utilized to increase the output torque and power capabilities of the electric motor-generator. Furthermore, depending on the torque and power demands, ambient temperatures and resulting thermal cooling requirements, the axial length needed for the axial flow fan could be quite substantial, thus reducing the axial length available for the electric motor-generator and significantly limiting its output torque and power capabilities.
Prior art on cooling of electric machines, such as disclosed in the example prior arts US 2012/0299403A1, US 2013/0028751A1, U.S. Pat. No. 4,904,891, EP 0244134B2, EP 0462647A1, and EP 0971131A2, have primarily described solutions that require using coolant fluid and internal or external fans for thermal cooling. These prior arts do not describe solutions that provide effective airflow and improved thermal cooling for electric machines, and particularly, electric machines operating in high ambient temperatures subject to stringent space constraints. The solution disclosed in the present invention uniquely differs from solutions on thermal cooling of electric machines in the prior arts in terms of the technique, constructional features of the shaft and layout of the electric machine that provide effective airflow and improved overall thermal cooling without using a fan, as well as improve the output torque and power capabilities of the electric machine.
Accordingly, the present invention describes a rotor shaft construction that provides effective airflow and improved cooling of a motor-generator, particularly belt-driven air-cooled motor-generator with stringent space constraints, without an external fan that utilizes axial space that can be effectively used to significantly increase the output torque and power capabilities of the motor-generator. The structure of the rotor shaft provides essential advantages over the current state-of-the-art in belt-driven air-cooled motor-generators that are cooled using axial flow fans. These advantages include: (1) providing effective airflow along both the hottest regions of the stator winding end-turns, through the inside of the rotor and axially along the shaft, through the bearing mounting surfaces on both ends of the housing, and thereby providing improved cooling of the entire motor-generator; and (2) enabling improved thermal performance margins which, in turn allow the motor generator to deliver relatively high output torque and power. In comparison, the axial flow fan provides airflow from one end of the motor-generator housing, over the stator windings and exhausts through the exit vents on the other end of the housing with margin airflow, if any, that reaches the rotor, shaft, and bearings mounted on both ends of the shaft.
Furthermore, it has been recognized by the authors of the present invention that the shaft construction when integrated into a belt-driven fan air-cooled motor-generator can yield improvement in output torque and power assist capabilities of the motor-generator that is directly related to the improved airflow and thermal cooling characteristics of the shaft construction.